It is well recognized that the Li-ion battery system has revolutionized our quality of life. We find the ubiquitous Li-ion battery in personal electronic devices, in hybrid electric vehicles, electric vehicles and the so-called hybrid power plants that leverage renewable energy sources to offset peak load. It is equally well known to those skilled in electrochemical energy storage systems that Li-ion batteries have limitations based on their inherent safety issues. Lithium itself has a relatively low melting point (180° C. at atmospheric pressure). More particularly, it is well understood that “post Li-ion” battery materials are essential to enabling long range electric vehicles and economic leveraging renewable energy generation such as lithium-sulfur, lithium-air, magnesium-sulfur and calcium-sulfur electrochemical cells.
As a commodity, lithium has reasonably low abundance compared to calcium and magnesium. Additionally, spent Li-ion batteries and packs have to be collected and processed to extract elements that pose risks to the environment such as elemental lithium, organic electrolyte, lithium salts, and various positive electrode elements such as nickel, manganese and cobalt. This creates an energy intensive cradle-to-grave process.
Now imagine an energy storage system that, when spent, is effortlessly processed back into the earth where the elemental form is so abundant that there is virtually no net negative environmental perturbation. Here we specifically focus on divalent metal-sulfur electrochemical cells (calcium and magnesium). Calcium in its native form (more specifically calcium carbonate and calcium oxide) is everywhere in appreciable amounts; in the water we drink, in concrete, in building materials that make up residential houses and commercial skyscrapers and as a primary constituent in the human body forming our skeletal system. Nonetheless, it's evident that calcium, in general, is more ubiquitous than lithium.
Futile efforts have ensued since the 1970's to investigate and control metallic calcium's activity in the form of an electrochemical cell. Researchers have essentially found that calcium is so reactive that it spontaneously generates an ionic insulating interface with nearly every trace element it in comes in contact with, rendering it useful only to a single discharge, primary battery with a shelf life of less than one day. Lithium and calcium based energy storage systems were competitively investigated in the late 1970's and throughout the 1980's until Goodenough and co-works achieved a breakthrough for the rechargeable Li-ion battery (U.S. Pat. Nos. 5,910,382, 6,514,640). This was the tipping point at which Li-ion battery material research dominated other energy storage platforms.
To establish the need of a functional coating on electrochemical energy storage electrodes, the following paragraphs will outline previous strategies and milestones relating to materials developments to achieve a lithium and calcium based secondary electrochemical energy storage system. The typical cell configuration consists of a counter electrode (CE), a metal salt dissolved in a nonaqueous, aprotic organic electrolyte usually in the concentration range 0.5M to 1.5M (EL), and a working electrode (WE).
Aurback and co-worker's concluded that metallic calcium is too reactive toward nonaqueous liquid electrolytes, forming a compact ionic insulating passivation layer (J. Eletrochem. Soc., 138, 1991, 3536). This layer is thought to be composed of calcium oxide, calcium carbonate and various calcium alkyl compounds inhibiting the electrochemical deposition of Ca2+. It was shown through their studies that the most promising electrolyte was 0.5M Ca(ClO4)2 in acetonitrile.
Amatucci and co-workers (J. Eletrochem. Soc., 148, 2001, A940) focused on a reversible intercalation cathode, V2O5, for di and trivalent metal ions. It was shown that Ca2+ can be electrochemically inserted and deinserted two-and-a-half times. However, metallic calcium CE was substituted with an activated carbon CE which can only accommodate an electrical double layer of Ca2+ a few times before breaking down.
More recently, Hayashi et. al. demonstrated Ca2+ insertion into crystalline V2O5 (WE) by using Ca metal (CE). Thus, the Ca2+ was unable to be de-inserted (Electrochem Solid St., 7, 2004, A119).
Even more recently, Kano et. al., proposed that calcium isopropoxide, as an EL additive to calcium di(bis(trifluoromethanesulfonylimide)) in propylene carbonate, is essential for calcium deposition (abstract #50, 218th Electrochemical Society Meeting, 2010). Although it is claimed in the text that more than one redox cycle can be achieved, it is not figuratively demonstrated. Several attempts were made to reproduce this result as a reference to the current invention, however all attempts failed. It was determined through cyclic voltammetry that calcium di(bis(trifluoromethanesulfonylimide)) decomposed against calcium metal forming a CaF2 side product. Moreover, platinum metal was used as the WE and CE in Kano's example thus they failed to demonstrate the stability of calcium di(bis(trifluoromethanesulfonylimide)) against calcium metal. To date, there has been no advancement or effort to directly stabilize the calcium-electrolyte interface. One of the inventive concepts herein takes use of a well-known Li-ion battery electrolyte additive, tris(pentafluorophenyl)borane, as a nonaqueous catalyst to synthesize a general class of well-defined binary hybrid siloxy derived resins. Another inventive concept herein applies hybrid siloxy derived resins as passivants on electrodes used in electrochemical energy storage cells.
The following paragraphs will describe the state-of-the-art of borosiloxane resin synthesis, complex polysiloxanes achieved by catalyzed hydrosilylation and their limitations.
Polysiloxanes are one of the most technologically important class of materials. They constitute one of the more broader platforms of root materials as a result of their ease of functionalization, attainable properties and topology and organic side group functionalization. Polysiloxanes are therefore the basis for diversifying a well-known, earth abundant platform to chemically and mechanically control the activity of highly reactive electrochemical interfaces to enable the forward progress in high energy density electrochemical energy systems.
The synthesis, thermal and mechanical properties and technological applications of polysiloxanes are well documented [1]. Much effort has focused on Lewis acid catalyzed condensation using tris(pentafluorophenyl)borane, B(C6F5)3. B(C6F5)3 has been found to be a robust, stable and water tolerant Lewis acid catalyst enabling many key organic transformations and polymerizations [2a,b]. Parks and co-workers first demonstrated the mild, selective hydrosilation of C═O functions by B(C6F5)3 [3]. Their work showed that aromatic aldehydes, ketones and esters could undergo astonishingly mild reductions at room temperature.
Many examples of reductive transformations utilizing B(C6F5)3 have followed. In particular, Rubinsztajn et al. demonstrated the catalytic synthesis of polysiloxane copolymers by the condensation reaction between hydrosilanes and alkoxysilanes [4]. This was the first example of an efficient and clean heterocondensation reaction between disilanes and dialkoxysilanes whereby the majority of the byproduct is autogenously removed as a low boiling alkane (i.e. methane, ethane, propane).
Most recently, the simplicity of accessing highly branched siloxanes and polysiloxane copolymers were reported. Thompson and Brook [5] described the assembly of complex 3-D siloxane architectures near ambient conditions producing symmetrically branched siloxane structures in very high yield. At the same time, Chojnowski et al [6], demonstrated polycondensation of tetraalkoxysilanes with 1,1,3,3-tetramethyldisiloxane catalyzed by B(C6F5)3 yielding highly branched organopolysiloxanes while Rubinsztajn et al. has shown synthesis of siloxane networks by the B(C6F5)3 catalyzed disproportionation of hydridosiloxanes [7a-b].
Siloxanes are thermally stable, generally inert but in the case of demanding applications, such as the electrolytic stress in an energy storage device, there is need for improvement and diversification. Siloxanes are neither stable in basic environments, nor when exposed to high temperatures—they are prone to thermal oxidation and rearrangement of the polymer backbone resulting in loss of desired properties.
The inventive concept herein extends Lewis acid catalyzed hydrosilylation to the silylation of metalloid, alkali and transition metals to yield highly pure, nonaqueous binary and ternary compositions. These compositions are most commonly synthesized by the sol-gel method to obtain binary or ternary siloxane derivatives.
For example, the synthesis of borosiloxane (═B—O—Si≡) has been demonstrated by the sol-gel process wherein hydrolysis and polycondensation of boric acid [8a,b] and trialkyl borates [9a-c] with alkoxysilanes and silanols yields ═B—O—Si≡ bridge formation [9a].
However, the limiting features of the this synthetic route are lack of purity, efficiency and control over fidelity, thus limiting high level molecular shaping resulting in less than optimal physical properties of these materials. The ability to make the ═B—O—Si≡ bridge in nonaqueous conditions at room temperature has never been reported to the knowledge of this inventor. Furthermore, published synthetic routes at elevated temperature are cumbersome and inefficient observed by the lack of diversified synthetic procedures in the literature.
In a typical sol-gel derived borosiloxane (═B—O—Si≡) resin, the final loading of B is significantly small with respect to Si owing to a boric acid thermodynamic sync [10]. In the densified oxide state with high boron loading, the predominant species observed is the ═B—O—B═ bridge, while the minor is ═B—O—Si≡. Moreover, the rate of hydrolysis is very slow, on the order of days to weeks.
Similarly, halosilanes can be reacted with alkoxyboranes in the presence of Lewis acids to yield polyborosiloxanes [11a,b]. However, the synthetic procedure described in WO2009/111193A1 is cumbersome insomuch it is multistep requiring hours of elevated temperature and several isolation and drying steps. Thus there is a continuing need to develop highly efficient, chemically controllable, environmentally benign and cost effective synthetic methods to produce functional, as-prepared, binary siloxanes like polyborosiloxane and ternary siloxanes of varying formulation therefrom. Of technical priority is the ability to produce a stable borosiloxane resin with controllable Si:B stoichiometry such that intrinsic thermal, adhesive, ion-conducting and bioactive potential can be exploited.